1. Field of the Invention
The present invention relates to a method for stabilizing an output of higher harmonic waves used in the fields of optical information processing, optical application measurement control, and the like utilizing coherent light, and a short wavelength laser beam source using the method.
2. Description of the Related Art
In the field of optical information processing, short wavelength laser beam sources for optical recording require an output of several mW or more. As blue laser beam sources, the combination of a semiconductor laser emitting fundamental waves and a light wavelength conversion device generating higher harmonic waves of the fundamental waves is promising.
FIG. 22 is a cross-sectional view showing a structure of a conventional short wavelength laser beam source 5000 emitting blue light. Fundamental waves P1 emitted by a semiconductor laser 121 are collimated by a collimator lens 124 and focused onto an optical waveguide 102 formed inside of a light wavelength conversion device 122 by a focus lens 125. The fundamental waves P1 are converted into higher harmonic waves P2 in the optical waveguide 102 and output. Each component of the short wavelength laser beam source 5000 is mounted on a base member 120 made of Al. The light wavelength conversion device 122 is positioned on a quartz plate 123 with its face having the optical waveguide 102 down.
Next, the light wavelength conversion device 122 used in the conventional short wavelength laser beam source 5000 will be described.
FIG. 23A is a perspective view of the conventional light wavelength conversion device 122; FIG. 23B is a cross-sectional view taken along a line 23B--23B of FIG. 23A. Hereinafter, the operation of the light wavelength conversion device 122 will be described by illustrating the generation of higher harmonic waves (wavelength: 437 nm) from fundamental waves (wavelength: 873 nm) (see K. Yamamoto and K. Mizuuchi, "Blue light generation by frequency doubling of a laser diode in a periodically-domain inverted LiTaO.sub.3 waveguide", IEEE Photonics Technology Letters, Vol. 4, No. 5, pp. 435-437, 1992).
As shown in FIGS. 23A and 23B, the light wavelength conversion device 122 includes the optical waveguide 102 formed in a LiTaO.sub.3 substrate 101. The optical waveguide 102 is provided with periodically domain-inverted layers (domain-inverted regions) 103. The mismatch in propagation constant between the fundamental waves P1 and the higher harmonic waves P2 to be generated is compensated by a periodic structure composed of the domain-inverted regions 103 and non-domain-inverted regions 104. This allows the fundamental waves P1 to be converted into the higher harmonic waves P2 at high efficiency so as to be output. The arrows in FIG. 23B represents the direction of a domain in each region.
Next, the principle of amplification of the higher harmonic waves in the light wavelength conversion device 122 will be described with reference to FIGS. 24A and 24B.
FIG. 24A schematically shows inner structures, namely, the direction of domains of a device 131 which have no domain-inverted regions and of a device 132 which has domain-inverted regions. The arrows in FIG. 24A represent the direction of a domain in each region.
In the device 131, domain-inverted regions are not formed and the directions of domains are aligned in one direction. When fundamental waves pass through the device 131, the waves are partially converted into higher harmonic waves. However, in the structure of the device 131, an output of higher harmonic waves 131a merely repeats increasing and decreasing along the passing direction of the optical waveguide, as shown in FIG. 24B.
On the other hand, in the device 132 which has first-order periodically domain-inverted regions, an output of higher harmonic waves 132a increases in proportion to the square of length L of the optical wavelength as shown in FIG. 24B. It should be noted that only when a quasi-phase match is established, the output of the higher harmonic waves P2 can be obtained from the incident fundamental waves P1 in the domain-inverted structure. The quasi-phase match is established only when a period .LAMBDA.1 of the domain-inverted region is identical with .lambda./(2(N2.omega.-N.omega.)), where N.omega. is an effective refractive index of the fundamental waves (wavelength: .lambda.), and N2.omega. is an effective refractive index of the higher harmonic waves (wavelength: .lambda./2).
A method for producing a conventional light wavelength conversion device 5000 having the above-mentioned domain-inverted structure as a fundamental structure component will be described.
First, a periodic Ta film pattern with a width of several .mu.m is formed on the LiTaO.sub.3 substrate 101 made of non-linear optical crystal by vapor deposition and photolithography. The Ta film pattern is subjected to a proton-exchange treatment at 260.degree. C., followed by being heat treated at around 550.degree. C. Thus, the domain-inverted regions 103 are formed in the LiTaO.sub.3 substrate 101. Then, a Ta film slit is formed on the LiTaO.sub.3 substrate 101, heat treated in pyrophosphoric acid at 260.degree. C. for 12 minutes, and subjected to an anneal treatment at 420.degree. C. for one minute. Thus, the optical waveguide 102 is formed.
When the optical waveguide 102 has a length of 10 mm and the fundamental waves P1 having a power of 37 mW with respect to a wavelength of 873 nm is input to the light wavelength conversion device 122 produced as described above, higher harmonic waves P2 having a power of 1.1 mW can be output.
However, allowable width of the light wavelength conversion device 122 with respect to the wavelength of the fundamental waves is generally as small as 0.1 nm. For this reason, the light wavelength conversion device 122 cannot allow mode hopping of a semiconductor laser and spreading of the wavelength of output light.
For example, in the conventional light wavelength conversion device 122 having the above-mentioned domain-inverted regions, the allowance with respect to the wavelength fluctuation of a fundamental wave laser beam at a device length of 10 mm is very narrow; typically, an allowable wavelength half value width of around 0.1 nm. The allowable change with respect to temperature is typically as small as 3.degree. C. Because of this, when a light wavelength conversion device is combined with a semiconductor laser, the following problems arise: The output of the semiconductor laser is likely to be affected by the change in temperature and consequently wavelength fluctuation occurs in output light; as a result, fundamental waves are not converted into higher harmonic waves or the output of higher harmonic waves converted from fundamental waves greatly fluctuates.
The above-mentioned problems will be described in detail below.
Typically, when the wavelength of a semiconductor laser shifts by only 0.05 nm, the output of higher harmonic waves to be obtained becomes half of an intended value. The allowability with respect to the change in wavelength of a semiconductor laser is small. For example, when the ambient temperature during the operation of a semiconductor laser shifts from 20.degree. C. to 21.degree. C., the vertical mode of the semiconductor laser shifts by one and the oscillation wavelength shifts from 820.0 nm to 820.2 nm. Because of this, the output of higher harmonic waves becomes zero.
Regarding the allowable width of the light wavelength conversion device 122 with respect to the change in temperature, when the ambient temperature changes, the output of higher harmonic waves cannot be obtained even if the oscillating wavelength of the semiconductor laser is stable. Furthermore, frequent occurrence of mode hopping causes noise leading to problems in reading from optical disks.